I think that the latter is do-able, but substitutions also fail where the app doesn't support them. In latter case the combination could be '_st ' substitutes to st. Or use the character substitution feature in OS X, similar to the fractions, for 'st', 'th', 'nd', or make specific characters for them and assign to the keyboard (in System Prefs>Languages&Text>Text). I wonder if it can be automated and that automation be put on a menubar? Say, script the baseline shift up and sizing reduce on a key or a process and reverse the procedure. (Note that the keystroke for superscript is command-control-+, but that actually means you have to push command-control-shift-= on a US English keyboard.) In Pages, there's a simple keystroke for each, and it is smart enough to adjust the font size for you. In TextEdit, this simply moves the baseline up or down, and you would then probably want to change the font size as well. In Apple's apps, you generally use Format -> Font -> Baseline -> Superscript/Subscript. I'll try to remember this long and complicated process.

Full blown word processors will require their own font management techniques ā lā Pages.

You just type away in certain applications- TextEdit, Mail or any RTF word processor (guess). Below that is the mapping scheme I devised. These provide some illustration of my favorites. Here are some fonts I tried with the new mapping. Also I found the substitute characters' shifted capitals look better than the lower case ones when displayed in several different fonts, No worries, I can live with that and take advantage of the substitutions. The only remaining problems are little character anomalies in certain fonts (Helvetica is shown) that show up funny with substitute characters. I got the character substitutions set up the way I like them.

Mac paint 2 text superscript